How do I get a python reference to the viewport camera?
If you use 'PRE_VIEW' or 'POST_VIEW' as draw handler type, you will be in world space and can draw straight ahead. If you want to draw on top of everything ("X-Ray"), then calculate the 2d region coordinates and use 'POST_PIXEL' - there is a utility function for the conversion:

from bpy_extras.view3d_utils import location_3d_to_region_2d


Here is a quick example that draws a line between 3D cursor and active object origin, using 'POST_VIEW', which will consider depth of other objects ('PRE_VIEW' would draw under everything else):

import bpy
import bgl
import blf


def draw_callback_px(self, context):

    ob = context.object
    if ob is None:
        return

    font_id = 0  # XXX, need to find out how best to get this.

    # draw some text
    blf.position(font_id, 15, 30, 0)
    blf.size(font_id, 20, 72)
    blf.draw(font_id, ob.name)

    # 50% alpha, 2 pixel width line
    bgl.glEnable(bgl.GL_BLEND)
    bgl.glColor4f(1.0, 1.0, 1.0, 0.5)
    bgl.glLineWidth(2)

    bgl.glBegin(bgl.GL_LINE_STRIP)
    bgl.glVertex3f(*ob.matrix_world.translation)
    bgl.glVertex3f(*context.scene.cursor_location)
    bgl.glEnd()

    # restore opengl defaults
    bgl.glLineWidth(1)
    bgl.glDisable(bgl.GL_BLEND)
    bgl.glColor4f(0.0, 0.0, 0.0, 1.0)


class ModalDrawOperator(bpy.types.Operator):
    """Draw a line with the mouse"""
    bl_idname = "view3d.modal_operator"
    bl_label = "Simple Modal View3D Operator"

    def modal(self, context, event):
        context.area.tag_redraw()

        if event.type in {'RIGHTMOUSE', 'ESC'}:
            bpy.types.SpaceView3D.draw_handler_remove(self._handle, 'WINDOW')
            return {'CANCELLED'}

        return {'PASS_THROUGH'}

    def invoke(self, context, event):
        if context.area.type == 'VIEW_3D':
            # the arguments we pass the the callback
            args = (self, context)
            # Add the region OpenGL drawing callback
            # draw in view space with 'POST_VIEW' and 'PRE_VIEW'
            self._handle = bpy.types.SpaceView3D.draw_handler_add(draw_callback_px, args, 'WINDOW', 'POST_VIEW')

            self.mouse_path = []

            context.window_manager.modal_handler_add(self)
            return {'RUNNING_MODAL'}
        else:
            self.report({'WARNING'}, "View3D not found, cannot run operator")
            return {'CANCELLED'}


def register():
    bpy.utils.register_class(ModalDrawOperator)


def unregister():
    bpy.utils.unregister_class(ModalDrawOperator)

if __name__ == "__main__":
    register()

    for area in bpy.context.screen.areas:
        if area.type == 'VIEW_3D':
            bpy.ops.view3d.modal_operator({'area': area}, 'INVOKE_DEFAULT')
            break